In the semiconductor epitaxial deposition process, too large lattice mismatch between epitaxial substrate and deposition material, such as gallium nitride (GaN) deposits on a silicon (Si) substrate, will yield intrinsic stress between two materials, and thereby curving the surface of the silicon substrate. Also, lattice mismatch between epitaxial layers in the epitaxial deposition process will yield bending the substrate. Besides, thermal mismatch between layers will yield bending during the fabricating process. The phenomena will reduce the epitaxial quality and affect the fabricating process, thereby increasing the fabricating costs. It results in a need to monitor the curvature variation of the substrate in real time during the epitaxial process and realize the most significant stress is in which layer structure immediately. Therefore, it becomes an important issue for the manufacturers to adjust and control corresponding process parameters to reduce the stress.